| − | During the latter stages of the luteal phase the corpus luteum begins to regress under the effect of PGF<sub>2α</sub>. Normally the concentrations of PGF<sub>2α</sub> in arterial blood is relatively low due to extensive metabolism via the lungs. However there is a large concentration gradient from the uterine vein to the ovarian artery resulting in higher concentrations of PGF<sub>2α</sub> than in systemic circulation. If a pregnancy is to remain viable then luteolysis needs to be avoided and this is achieved where concentrations of PGF<sub>2α</sub> remain below a threshold level allowing the corpus luteum to continue to secrete prostaglandin. There are two main factors involved in the regulation of uterine secretions of PGF<sub>2α</sub>; oxytocin secretions from the corpus luteum and molecules secreted by the developing embryo that facilitate the maternal recognition of pregnancy. | + | During the latter stages of the luteal phase the corpus luteum begins to regress under the effect of PGF<sub>2α</sub>. Normally the concentration of PGF<sub>2α</sub> in arterial blood is relatively low due to extensive metabolism via the lungs. However there is a large concentration gradient from the uterine vein to the ovarian artery resulting in higher concentrations of PGF<sub>2α</sub> than in systemic circulation. If a pregnancy is to remain viable then luteolysis needs to be avoided and this is achieved where concentrations of PGF<sub>2α</sub> remain below a threshold level allowing the corpus luteum to continue to secrete prostaglandin. There are two main factors involved in the regulation of uterine secretions of PGF<sub>2α</sub>; oxytocin secretions from the corpus luteum and molecules secreted by the developing embryo that facilitate the maternal recognition of pregnancy. |